A. c. corona charging apparatus

ABSTRACT

A method of increasing the current flow from an a.c. corona charging apparatus and the apparatus therefor are provided in accordance with the teachings of the present invention. According to one embodiment of this invention a.c. corona charging apparatus is provided with a corona discharge electrode and a dielectric shield disposed in partially surrounding relationship with respect to said corona discharge electrode. Conductive means is mounted on the external surface of said dielectric shield such that said conductive shield is contiguous with and collateral with said dielectric shield. The conductive means is adapted to be supplied with a reference potential such that when a corona generating a.c. voltage is applied to said corona discharge electrode an a.c. corona current is produced having a magnitude significantly in excess of that heretofore obtainable.

United States Patent [191 Parker June 26, 1973 A. C. CORONA CHARGINGAPPARATUS Delmer G. Parker, Rochester. NY.

[73] Assignee: Xerox Corporation, Stamford, Conn.

[22] Filed: Apr. 21, 1971 211 App]. No.: 136,125

[75] Inventor:

Primary Examiner-James W. Lawrence Assistant Examiner-C. E. ChurchAttorney-James J. Ralabate, Albert A. Mahassel, Michael J. Colitz, TerryJ. Anderson and Marn &

.langarathis [57] ABSTRACT A method of increasing the current flow froman a.c. corona charging apparatus and the apparatus therefor areprovided in accordance with the teachings of the present invention.According to one embodiment of this invention a.c. corona chargingapparatus is provided with a corona discharge electrode and a dielectricshield disposed in partially surrounding relationship with respect tosaid corona discharge electrode. Conductive means is mounted on theexternal surface of said dielectric shield such that said conductiveshield is contiguous with and collateral with said dielectric shield.The conductive means is adapted to be supplied with a referencepotential such that when a corona generating ac. voltage is applied tosaid corona discharge electrode an ac. corona current is produced havinga magnitude significantly in excess of that heretofore obtainable.

7 Claims, 4 Drawing Figures PATENTEUJUNZS ms 3, 742.237

snm 1 0f 2 Fig. 2.

INVEN TOR.

Delmer G. Parker m' PATENIEDmuzs Ian SIEUZBFZ ATTORNEYS 1 A. c. CORONACHARGING APPARATUS This invention relates to a.c. corona chargingapparatus devices and in particular, to a method of and apparatus forincreasing the corona current flow from a.c. corona charging apparatushaving a dielectric shield.

In the electrophotographic reproducing art, it is necessary to deposit auniform layer of electrostatic charges on the surface of a photoreceptorsuch that the electrostatic charges may be selectively dissipated inaccordance with modulated radiation imaged thereon to form anelectrostatic latent image of an original document. The electrostaticlatent image will then be developed and the developed image may betransferred to a support surface to form a final copy of the originaldocument. If the photoreceptor comprises a conventional reusableelectrophotographic member, the photoreceptor may be cleaned andprepared for subsequent operations thereon. The foregoing has beendescribed in detail in U. S. Pat. No. 2,297,691 which issued to ChesterF. Carlson.

The prior art has suggested various techniques and devices for applyinga uniform electrostatic charge on the surface of a photoreceptor. Someof these techniques include: an alpha-emitting radioactive source forionizing the air by emitting alpha particles; a conductive rubber rollerhaving a potential applied thereto, which roller can be rolled over thephotoreceptor; a charged insulator placed in contact with thephotoreceptor whereby charges may be transferred from the insulator tothe photoreceptor; and a corona discharge device of the type describedin U. S. Pat. No. 2,777,957 which issued to L. E. Walkup. Although eachof the foregoing is applicable in specialized uses it is preferred todeposit the uniform electrostatic charge on the surface of aphotoreceptor by employing a corona discharge. A particular type ofcorona discharge device that has been readily included in conventionalelectrophotographic reproducing apparatus and is described in detail inU. S. Pat. No. 2,836,725 which issued R. G. Vyverberg and assigned toXerox Corporation. Corona charging apparatus is usually comprised of acorona discharge electrode, such as a corona wire, surrounded by aconductive shield. The corona discharge electrode is adapted to besupplied with a dc. voltage of sufficient magnitude to create a coronacurrent flow from the electrode to the surface of a photoreceptor inspaced registration therefrom. The geometry of typical corona chargingapparatus admits of various configurations as disclosed in U. S. Pat.No. 2,879,395 which issued to L. E. Walkup and assigned to XeroxCorporation.

Although corona charging apparatus is advantageously utilized to deposita uniform layer of electrostatic charge on the surface of aphotoreceptor various other applications thereof have been adopted.Typical of these applications are electrostatic transfer of a developedimage to a support surface, removal of background toner particles from adeveloped electrostatic latent image, and pre-cleaning a photoreceptorby neutralizing the charge on toner particles adhering to the surface ofthe photoreceptor after transfer of the developed image to a supportsurface. An attendant disadvantage of corona charging apparatusdeveloped by the prior art for use in electrophotographic reproducingdevices is the accumulation of dust particles and toner particles on andabout the interior of corona charging apparatus to such an extent thatthe corona current generated thereby substantially decreases as thedensity of particle accumulation increases. Accordingly, self-cleaningcorotrons have been developed which employ corona winds inherentlygenerated by corona charging apparatus as a means to clean the coronadischarge electrode and the interior walls of the surrounding shield.Detailed descriptions of such devices may be found in U. S. Pat. No.3,324,291 which issued to F. W. Hudson on June 6, 1967 and assigned toXerox Corporation and in U. S. Pat. No. 3,471,695 which issued to F. W.Hudson et al. on Oct. 7, 1969 and assigned to Xerox Corporation.

An alternative embodiment of corona charging apparatus self-cleaningcomprises an a.c. corona charging apparatus wherein the corona dischargeelectrode thereof is supplied with a corona generating a.c. voltage. Asis understood by those of ordinary skill in the art the creation of acorona current is predominantly determined by the potential differencebetween the corona discharge electrode and the partially surroundingshield. Accordingly, if the shield is comprised of a metal shieldsupplied with a reference potential such as ground potential, a positivecorona current will be generated when the difference between the a.c.voltage applied to the corona discharge electrode and the referencepotential supplied to the metal shield exceeds the positive coronathreshold voltage; and a negative corona current will be generated whenthe difference between the a.c. voltage applied to the corona dischargeelectrode and the reference potential supplied to the metal shieldexceeds the negative corona threshold. In theory then, a maximum a.c.corona current is generated for the embodiment of an a.c. coronacharging apparatus including a grounded metal shield. Unfortunately thegrounded shield a.c. corona charging apparatus suffers from thedisadvantage that toner particles or dirt particles which accumulate onthe inner walls of the metal shield and are not effectively removedtherefrom produce deleterious effects. More specifically, theseparticles are comprised of dielectric material which store the chargedions communicated thereto from the corona discharge electrode duringcorona discharge. As charge buildup on these particles occurs, a voltageis induced on the contaminated inner wall of the surrounding shieldresulting in a nonuniform potential difference between the coronadischarge electrode and the shield. Hence, the a.c. corona current issubject to variations due to the discontinuities in the aforementionedpotential difference.

A suggested improvement over the grounded metal shield a.c. coronacharging apparatus is comprised of an a.c. corona charging apparatushaving an insulating or dielectric shield constructed of a plasticmaterial such as Teflon or Mylar. The accumulation of dielectricparticles on the inner surface of the dielectric shield has noappreciable effect upon the a.c. corona current. This advantageouscharacteristic is however, achieved at the loss of magnitude of the a.c.corona current. The loss in corona current is brought about by the rapidincrease of voltage induced on the inner surface of the dielectricshield, thereby limiting the potential difference between the coronadischarge electrode and the dielectric shield to an undesirable range.In fact the rate of increase in the voltage induced on the inner surfaceof the dielectric shield closely approximates the rate of increase ofthe corona generating a.c. voltage applied to the corona dischargeelectrode. This may be explained by recognizing that the voltage inducedon the shield is directly proportional to the charge thereon andinversely proportional to the capacitance between the inner surface ofthe shield and ambient ground potential. Thus as the charge stored onthe shield increases, the voltage induced on the inner surface thereofincreases, and if the capacitance between the inner surface of theshield and ambient ground is a low value, a small buildup of chargeinduces a large voltage. Ambient ground potential is equivalent to oneplate of a parallel plate capacitor spaced a sufficient distance fromthe inner surface of the dielectric shield, which inner surface may beconsidered to be the other plate of the parallel plate capacitor, suchthat the capacitance therebetween is a low value. Consequently, a smallvalue of a.c. corona current induces a large voltage on the innersurface of the dielectric shield such that the potential differencebetween the corona discharge electrode and the surface of the dielectricshield is much less than the potential difference between the coronadischarge electrode and the surface of a grounded metal shield, therebymaintaining the corona current of the former corona charging apparatusat an undesirably lower magnitude than the corona current of the lattercorotron.

Therefore it is an object of the present invention to provide a methodof and apparatus for increasing the corona current of a.c. coronacharging apparatus.

It is another object of the present invention to provide self-cleaningcorona charging apparatus having an improved corona current output.

It is a further object of this invention to provide a method of andapparatus for increasing the corona current generated by a coronadischarge device, which device is relatively unaffected by theaccumulation of foreign particles thereon.

Various other objects and advantages of the invention will become clearfrom the following detailed description of an exemplary embodimentthereof and the novel features will be particularly pointed inconnection with the appended claims.

In accordance with this invention, a method of increasing the currentflow from the corona discharge electrode of an a.c. corona chargingapparatus, and the apparatus therefor, is provided wherein conductivemeans is disposed about the external surface of a partially surroundingdielectric shield, which conductive means is collateral with saiddielectric shield; a reference potential is applied to said conductivemeans; and a corona generating a.c. voltage is supplied to the coronadischarge electrode of said corona charging apparatus.

The invention will be more clearly understood by reference to thefollowing detailed description of an exemplary embodiment thereof inconjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a.c. corona charging apparatus inaccordance with the present invention;

FIG. 2 is a sectional view taken along lines 22 of FIG. 1;

FIG. 3 is a graphical representation of waveforms generated by apparatusof the prior art; and

FIG. 4 is a graphical representation of the waveforms generated by theapparatus of the present invention.

Referring now to the drawings wherein like reference numerals are usedthroughout, and in particular to FIG.

1, there is illustrated a perspective view of an a.c. corona dischargedevice in accordance with the present invention which comprises a coronadischarge device 10 including a shield of dielectric material 12, alayer of conductive material 11, insulating end plates 14 and 15, and acorona discharge electrode 13. The shield of dielectric material 12 isdisposed in partially surrounding relationship with respect to thecorona discharge electrode 13 and admits of rectangular cross section.The dielectric material may be comprised of Teflon, Mylar, Lucite,Plexiglas, Lexan or the like. As is illustrated in FIG. 1, and in moredetail in FIG. 2, the dielectric shield 12 is formed of a pair ofparallel side walls maintained in spaced apart relationship by an upperinterconnecting wall. The dielectric shield 12 may be of unitaryconstruction and defines an opening 16 at the lower portion thereof. Thelongitudinal dimension of dielectric shield 12 is coextensive with thelongitudinal dimension of corona discharge electrode 13. The thicknessof the dielectric shield 12 is determined by the dielectric strength ofthe material used, the frequency of the corona generating a.c. voltage,and the diameter of the toner particles that might adhere to the wallsof the dielectric shield. Satisfactory experiments have been conductedwith a dielectric shield having a thickness of one mil, and a reasonablerange of thickness is between 0.25 and 10 mils. The dielectric materialtends to break down below 0.25 mils and, as will soon be described, manyof the advantages obtained by the present invention are minimized if thethickness of the dielectric material is substantially in excess of 10mils.

A layer of conductive material 11 is mounted on the external surface ofthe dielectric shield 12 so as to be contiguous and collateral with saiddielectric shield. The conductive material 11 may comprise a unitarystructure such as a foil sheet or the conductive material 11 maycomprise a conductive paint, strips of conductive material, a wire gridnetwork or the like. The thickness of the conductive material 11 is notcritical per se as will soon be understood from the forthcomingdescription, and therefore, may provide the structural rigidity for thecorona discharge device of the present invention. Although notillustrated herein, the upper interconnecting wall of the dielectricshield 12 and the corresponding portion of the contiguous conductivematerial 11 may be provided with elongated openings substantiallyparallel to the corona discharge electrode 13 to act as an exit orificefor the flow of corona air currents inherently generated by the coronadischarge device 10 as described in aforementioned U. S. Pat. Nos.3,324,291 and 3,471,695.

The corona discharge electrode 13 is mounted within insulating endportions Ml and 15 disposed at opposite ends of the shield structurecomprised of the dielectric shield 12 and the contiguous layer ofconductive material lll. Suitable connecting means 17 are provided onthe end portions 14 and 15 for connecting the corona discharge electrode13 to a suitable source of corona generating a.c. voltage, not shown.The corona discharge electrode 13 may comprise one or a plurality offine wires elongatedly disposed between the opposite end portions 14 and15. A typical wire that may be utilized is a 3.5 mil diameter platinumalloy wire, well known to those of ordinary skill in the art.

As illustrated in the sectional view of FIG. 2, the conductive material11 may be provided with suitable connecting means 21 for applying areference potential such as a dc. voltage or, if desired, groundpotential to the conductive material 11. The corona discharge devicethus formed is disposed in spaced registration from the surface of aphotoreceptor 22 in such manner that relative motion may be providedbetween the corona discharge device and the photoreceptor.

The operation of the corona discharge device in accordance with thepresent invention will now be described in conjunction with FIGS. 3 and4. An a.c. corona current will be generated from the corona dischargeelectrode 13 when the a.c. voltage applied thereto exceeds the coronathreshold voltage. For pur poses of explanation it will be assumed thatthe a.c. voltage applied to the corona discharge electrode 13 issinusoidal and the frequency thereof is 60 Hz. However it should beunderstood that other frequencies may be utilized and in fact the coronadischarge device 10 of the present invention has been satisfactorilyoperated with frequencies on the order of 600 Hz. During each positionhalf cycle, a positive corona current flows from the corona dischargeelectrode 13 to the surface of the photoreceptor means 22 and duringeach negative half cycle a negative corona current flows from the coronadischarge electrode 13 to the surface of the photoreceptor means 22.Corona current additionally flows from the electrode 13 to the walls ofthe dielectric shield 12. Inasmuch as the shield 12 is comprised ofdielectric material, the flow of corona current thereto will result in abuildup of charge on the inner surface of the walls of shield 12.Consequently, a voltage will be induced on the inner surface of theshield 12. When the voltage induced on the walls of shield 12 obtains avalue such that the potential difference between the voltage applied tothe electrode 13 and the induced voltage is equal to or less than thecorona threshold voltage, the corona emission of the electrode 13 willterminate. Stated otherwise, the electric field intensity near thesurface of the corona discharge electrode 13 is determined by thepotential difference between the voltage applied to the electrode 13 anda voltage induced on the walls of shield 12. A greater electric fieldintensity produces a greater corona current. The present inventionincreases the corona current flow from the corona discharge electrode 13by maintaining a higher electric field intensity at the surface of thecorona discharge electrode 13 than has been heretofore thought possiblein corona discharge devices having a dielectric shield.

The interior surface of the dielectric shield 12 comprises one plate ofa grounded parallel plate capacitor, the other plate being comprised ofthe grounded conductive material 11. As is understood, the capacitanceof this grounded parallel plate capacitor is a function of the thicknessof the dielectric shield 12 and the dielectric constant thereof. Thevoltage induced on the inner surface of the dielectric shield 12 isdetermined by the capacitance of the aforementioned grounded parallelplate capacitor and may be represented as V Q/C ,where V is the voltageinduced on the inner surface of the wall of the dielectric shield, Q isthe instantaneous charge stored on the inner surface of the wall of thedielectric shield 12, and C is the capacitance of the grounded parallelplate capacitor. If the capacitance of the grounded parallel platecapacitor is large, it is understood that a substantial amount of chargemust be stored on the inner surface of the wall of the dielectric shield12 before the induced voltage V is sufficient to cause termination ofthe corona current. Conversely, if the capacitance of the groundedparallel plate capacitor is small, a relatively small amount of storedcharge will result in an induced voltage V, whereby the corona currentwill be terminated. Thus, the provision of continuous conductivematerial 11 supplied with a reference potential such as ground potentialtends to increase the magnitude during each half cycle, of the a.c.corona current emitted by the corona discharge electrode 13. It isapparent that the flow of corona current to the surface of photoreceptor22 will result in a buildup of charge thereon which tends to decreasethe potential difference between the voltage applied to corona dischargeelectrode 13 and the voltage induced on the photoreceptor 22 whereby thecorona current flow is decreased. However, it may be assumed that thephotoreceptor 22 is transported at a rate sufficient to inhibit thecharge retained on the surface thereof to reach a critical value whichsubstantially affects the corona current.

The increase in corona current flow in accordance with the presentinvention may best be appreciated by comparing the operation of thecorona discharge device 10 illustrated herein with the operation ofprior art corona discharge devices. A typical prior art a.c. coronadischarge device includes a shield such as dielectric shield 12 of thepresent invention that is not provided with a metallic layer contiguoustherewith. When, at time t the a.c. voltage V, applied to the coronadischarge electrode of the prior art device increases to a valueexceeding the corona threshold voltage V,, the electric field intensityat the surface of the corona discharge electrode is of sufficientmagnitude such that corona emission occurs and a corona current I,begins to flow. The corona current flows to the surface of aphotoreceptor and in addition, a corona current path is establishedbetween the corona discharge electrode and the inner surface of thewalls of the dielectric shield. Consequently, the charge stored on theinner surface of the walls of the dielectric shield increases and anincreasing voltage is induced on the shield. The inner surface of thesurface of the wall of the dielectric shield comprises one plate of agrounded plate capacitor, the other plate being comprised of the ambientor virtual ground potential in the vicinity of I the dielectric shield.Ambient ground may be represented by a grounded plate disposedexternally of the dielectric shield and parallel to and substantiallyspaced from the inner surface of the wall of the shield. As isunderstood, the large separation between the plates of the groundedparallel plate capacitor thus formed results in a low capacitancethereof. Hence the voltage induced on the shield may be represented as V(l/C) I l d! where V is the voltage induced on the shield, I is thecorona current, and C is the capacitance of the grounded parallel platecapacitor. It may be observed from FIG. 3 that a small increase in thecorona current results in a correspondingly small increase in the chargestored in the inner surface of the wall of the dielectric shield becauseof the small capacitance of the grounded parallel plate capacitor, but alarge increase in the induced voltage on the walls of the shield. Infact, the voltage V induced on the walls of the shield changes at a ratethat is approximately equal to the rate of change of the a.c. voltage Vapplied to the corona discharge electrode.

The voltage V,,, induced on the walls of the shield rapidly increases toa value such that the potential difference between the a.c. voltage V,supplied to the corona discharge electrode and the voltage V,,, is equalto the corona threshold voltage V This occurs in approximate coincidencewith the positive peak voltage of the a.c. voltage V,,. Hence as V,decreases the potential difference between V and V,,, will be less thanthe corona threshold voltage V,.. Because corona discharges exhibit ahysteresis effect, corona current I continues to flow until time t atwhich time the corona current I is terminated. Termination of the coronacurrent I prevents further buildup of charge on the inner surface of thewalls of the dielectric and accordingly, V retains its peak magnitudeuntil time 1 At this time the potential difference between the a.c.voltage V applied to the corona discharge electrode and the peak voltageV stored on the walls of the shield is equal to the negative coronathreshold voltage -V Consequently, the electric field intensity at thesurface of the corona discharge electrode is of sufficient magnitude tocreate a corona emission and negative corona current I begins to flow.The negative corona current produces a buildup of negative charge on theshield thereby inducing an increasingly negative voltage V,,, on theinner surface of the walls of the shield. Since the capacitance of thegrounded parallel plate capacitor comprised of the inner surface of thewalls of the shield and ambient ground potential is of a low value thevoltage V,,, decreases at a rate that approximates the rate of change ofvoltage V,,. When the a.c. voltage V, obtains its negative peak value,the potential difference between the voltage V, and the voltage V,,, isequal to the negative corona threshold voltage -V,. As the a.c. voltageV,, tends to increase the potential difference between V and V,,, fallsbelow the negative corona threshold volt age. The hysteresis effect,however, enables the corona current I to flow until time t, when thecorona current I, terminates. The voltage V induced on the inner surfaceof the wall of the dielectric shield retains its negative peak valueuntil time whereupon the potential difference between the a.c. voltageV, and V,,, is equal to the corona threshold voltage V Consequentlycorona current I begins to flow and the foregoing cycle is repeated.

Referring now to FIG. 4, it is observed that the corona discharge device10 of the present invention operates in a manner similar to that justdescribed, however significant improvements are noted. The inner surfaceof the walls of dielectric shield 12 and the layer of groundedconductive material 11 contiguous therewith comprise first and secondplates of a parallel plate capacitor. The separation of the parallelplates, which is equal to the thickness of the dielectric shield 12 issufficiently small so that the capacitance is large. Conse quently, therate of increase of the voltage V induced on the inner surface of thewalls of the shield 12 from time t to time t is less than the rate ofchange of the a.c. voltage V,, applied to the corona discharge electrode13. Hence the potential difference between V,, and V in FIG. 4 duringthe interval t, to t is greater than the potential difference between V,and V in FIG. 3 during the corresponding interval t, to resulting in acorona current I of greater magnitude. In addition, it is seen that inthe present invention the positive half cycle of corona current I isterminated at a time 1' which is later than the time at which thepositive half cycle of corona current of the prior art is terminated.Similarly the magnitude and duration of the negative half cycle of thecorona current l in accordance with the present invention is greaterthan the magnitude and duration of the negative half cycle of coronacurrent I of the prior art. The increased corona current flow of thepresent invention is attributed to the increase in capacitance from theinner surface of the wall of dielectric shield 112 to ground. Thus, alarge increase in the corona current is required for a correspondinglylarge increase in the induced voltage V, on the walls of the shield inaccordance with the equation V f I dt/ C). Another advantage of thepresent invention is that the magnitude of the corona generating a.c.voltage applied to the corona discharge electrode 13 need not be asgreat as heretofore required in order to obtain a corona current ofequivalent magnitude.

The average d.c. value of the corona current emitted from the coronadischarge electrode 13 may be determined by applying a d.c. referencepotential of constant magnitude to contact 211 of conductive materialI 1. This modification of the corona discharge device of the presentinvention is desirable if the corona discharge device is to be utilizedto deposit a uniform layer of electrostatic charge of a given polarityon the surface of the photoreceptor 22. The geometry of the coronadischarge deviceltl of the present invention has been illustrated hereinas exhibiting a rectangular cross section. Typical dimensions are awidth of seveneighths inches, a height of five-eighths inches and alength of 10 inches. The corona discharge device, as illustrated in FIG.2, may be positioned 0.1 inch above the surface of the photoreceptor 22,and corona discharge electrode 13 may be spaced 0.25 inch from thesurface of the photoreceptor 22. The present invention, however, is notlimited to the aforementioned configuration and readily admits ofvarious shapes such as those disclosed in US. Pat. No. 2,879,395 whichissued to L. E. Walkup on Mar. 24, 1959 and assigned to XeroxCorporation. Thus the present invention exhibits desirablecharacteristics of corona discharge devices having a grounded metallicshield, i.e., increased corona current flow, and desirablecharacteristics of corona discharge devices having a dielectric shield,i.e., the operation thereof is relatively unaffected by the accumulationof toner particles or dust particles.

While the invention has been particularly shown and described withreference to a specific embodiment thereof, it will be obvious to thoseskilled in the art that the foregoing and various other changes andmodifications inform and details may be made without departing from thespirit and scope of the invention. It is therefore intended that theappended claims be interpreted as including all such changes andmodifications.

What is claimed is:

1. A method of increasing the current flow from the corona dischargeelectrode of an a.c. corona charging apparatus having a dielectricshield partially surrounding said corona discharge electrode and asurface, comprising the steps of:

providing a conductive layer in contact with the external surface ofsaid dielectric shield, said conductive layer being collateral with saiddielectric shield;

applying a reference potential to said conductive layer; and

supplying said corona discharge electrode with a corona generating a.cvvoltage.

2. The method of claim 1 wherein said step of applying a referencepotential to said conductive layer comprises applying ground potentialto said conductive layer.

3. The method of claim 1 wherein said step of applying a referencepotential to said conductive layer com prises applying a dc. voltageexhibiting a constant magnitude to said conductive layer.

4. An a.c. corona discharge device, comprising:

an elongated corona discharge electrode;

a dielectric shield disposed in partially surrounding relationship withrespect to said elongated corona discharge electrode, said dielectricshield being coextensive with said elongated corona discharge electrode;

a layer of conductive material in contact with the external surface ofsaid dielectric shield, said conductive means being collateral with saiddielectric shield;

means for applying a reference potential to said conductive means; and

means for supplying said elongated corona discharge electrode with acorona generating a.c. voltage.

5. Apparatus for applying an a.c. corona current to a surface,comprising:

corona discharge electrode means supported in spaced registration fromsaid surface;

a dielectric shield disposed in partially surrounding relationship withrespect to said corona discharge electrode means such that a coronacurrent path is defined between said corona discharge electrode meansand said surface, the interior surface of said dielectric shieldcomprising the first plate of a parallel plate capacitor;

conductive means in contact with the exterior surface of said dielectricshield and collateral therewith, said conductive means comprising thesecond plate of a parallel plate capacitor;

means for applying a reference voltage to said conductive means; and

means for applying a corona generating a.c. voltage to said coronadischarge electrode means.

6. Apparatus for applying an a.c. corona current to a surface,comprising:

corona discharge electrode means;

parallel plate capacitance means disposed in partially surroundingrelationship with respect to said corona discharge electrode means, saidparallel plate capacitance means including a first plate comprised ofthe interior surface of a dielectric shield and a second plate comprisedof conductive means in contact with the exterior surface of saiddielectric shield;

means for applying a reference voltage to said conductive means; and

means for applying a corona generating a.c. voltage to said coronadischarge electrode means whereby an a.c. corona current is generatedwhen the difference between the corona generating a.c. voltage appliedto said corona discharge electrode means and the voltage induced on thefirst plate of said parallel plate capacitance means exceeds the coronathreshold voltage.

7. The apparatus of claim 6 wherein the separation between said firstand second plates of said parallel plate capacitance means is from about0.25 to 10 mils whereby the capacitance of said parallel platecapacitance means is such that the rate of change of the voltage inducedon the first plate of said parallel plate capacitance means is less thanthe rate of change of the corona generating a.c. voltage applied to saidcorona discharge electrode means.

1. A method of increasing the current flow from the corona dischargeelectrode of an a.c. corona charging apparatus having a dielectricshield partially surrounding said corona discharge electrode and asurface, comprising the steps of: providing a conductive layer incontact with the external surface of said dielectric shield, saidconductive layer being collateral with said dielectric shield; applyinga reference potential to said conductive layer; and supplying saidcorona discharge electrode with a corona generating a.c. voltage.
 2. Themethod of claim 1 wherein said step of applying a reference potential tosaid conductive layer comprises applying ground potential to saidconductive layer.
 3. The method of claim 1 wherein said step of applyinga reference potential to said conductive layer comprises applying a d.c.voltage exhibiting a constant magnitude to said conductive layer.
 4. Ana.c. corona discharge device, comprising: an elongated corona dischargeelectrode; a dielectric shield disposed in partially surroundingrelationship with respect to said elongated corona discharge electrode,said dielectric shield being coextensive with said elongated coronadischarge electrode; a layer of conductive material in contact with theexternal surface of said dielectric shield, said conductive means beingcollateral with said dielectric shield; means for applying a referencepotential to said conductive means; and means for supplying saidelongated corona discharge electrode with a corona generating a.c.voltage.
 5. Apparatus for applying an a.c. corona current to a surface,comprising: corona discharge electrode means supported in spacedregistration from said surface; a dielectric shield disposed inpartially surrounding relationship with respect to said corona dischargeelectrode means such that a corona current path is defined between saidcorona discharge electrode means and said surface, the interior surfaceOf said dielectric shield comprising the first plate of a parallel platecapacitor; conductive means in contact with the exterior surface of saiddielectric shield and collateral therewith, said conductive meanscomprising the second plate of a parallel plate capacitor; means forapplying a reference voltage to said conductive means; and means forapplying a corona generating a.c. voltage to said corona dischargeelectrode means.
 6. Apparatus for applying an a.c. corona current to asurface, comprising: corona discharge electrode means; parallel platecapacitance means disposed in partially surrounding relationship withrespect to said corona discharge electrode means, said parallel platecapacitance means including a first plate comprised of the interiorsurface of a dielectric shield and a second plate comprised ofconductive means in contact with the exterior surface of said dielectricshield; means for applying a reference voltage to said conductive means;and means for applying a corona generating a.c. voltage to said coronadischarge electrode means whereby an a.c. corona current is generatedwhen the difference between the corona generating a.c. voltage appliedto said corona discharge electrode means and the voltage induced on thefirst plate of said parallel plate capacitance means exceeds the coronathreshold voltage.
 7. The apparatus of claim 6 wherein the separationbetween said first and second plates of said parallel plate capacitancemeans is from about 0.25 to 10 mils whereby the capacitance of saidparallel plate capacitance means is such that the rate of change of thevoltage induced on the first plate of said parallel plate capacitancemeans is less than the rate of change of the corona generating a.c.voltage applied to said corona discharge electrode means.